In many areas of optical imaging, particularly in the production of integrated circuits and flat panel displays using microlithography as well as in optical data storage, there is a desire to reduce the size of the image features. The resolution of an imaging system is given by the equation k .lambda./NA where .lambda. is the wavelength of the light, NA is the numerical aperture of the optical system, and k is a constant depending mainly on the type of material being imaged. Typical values of k are between 0.5 and 1. Today's system for microlithography used very short wavelength (down to 200 nm) and very large NA (up to 0.6) thus the potential for increased resolution, using optical means, is limited. Optical data storage devices are limited to wavelengths available from laser diodes therefore cannot go down below 400 nm at the present time. The main object of this invention is to image features smaller than possible using prior art methods and particularly smaller than predicted by the above equation for a given material. In this disclosure the word "light" should be interpreted as any energy beam, visible or not. In this context "light" is any radiation from x-rays to infrared, including electron beams and ion beams and imaging using magnetic or electric fields, such as magnetic data storage. Recording material in this disclosure means any material which reacts to an imagewise exposure by an energy beam such as film, photoresist, thermoresist, optical data storage coatings etc. The main emphasis of the invention is light, and in particular short wavelengths of light such as blue and UV, since it generates high resolution images.
Recording materials can be generally divided into two groups: those responding to exposure and those responding to the instantaneous value of the light (or radiation). Exposure is defined in optics as the integral of the light intensity over time: E=.intg. Idt. Traditional imaging materials such as photographic films or photoresists respond to exposure. When exposure reaches a threshold value a permanent change occurs in the material; for example a photoresist will change its solubility in the developer. Since these materials respond to exposure, a light intensity I for a duration of t will give the same exposure as 10I for a period of 0.1t (10I.times.0.1t=It). This is also know as the "law of reciprocity". Once the exposure reached the threshold value the change is normally permanent but it does not have to be permanent. Many materials are erasable or reversible. Since the exposure is a linear function of both I and t, the principle of linear superposition also holds for these materials. This principle states that .function.(a+b)=.function.(a)+.function.(b); in other words if an image is separated into two components, "a" and "b" and they are imaged separately the result will be identical to imaging them together as a single image of "a +b". Exposure which did not reach threshold may not have visible effects but it is stored in the material and will combine with any future exposure.
There are classes of imaging materials not obeying the "reciprocity law " or linear superposition of images. The best known are thermal imaging materials. For example, optical data storage materials such as the well known Recordable Compact Disk (CD-R) respond to the peak temperature reached by the laser writing spot. When the peak temperature exceeds a threshold value, a change (permanent or reversible) occurs. An exposure to a lower temperature has no effect as the heat will dissipate and material will return to room temperature and will have no "memory" of partial exposures. True thermal recording materials cannot obey the "law of reciprocity" since they need to be exposed to the room temperature for extended periods of storage and no accumulated effect is allowed. It is easy to see why a recording material based on a physical change, such as melting or ablation, will not follow the reciprocity law. Many materials based on a chemical change can also be engineered not to follow the reciprocity law and respond only to the instantaneous value of the light or radiation. Chemical reactions, in general, double in speed for a temperature increase of 10.degree. C. A chemical reaction can progress a billion times faster at 325.degree. than at 25.degree. C., as 2 exp (325-25).apprxeq.10 exp 9. Such a chemical reaction will appear to have a sharp threshold temperature. Any heating below this threshold will not have a permanent effect, as the amount of materials reacting will be very small. Since it is easy to raise the temperature of a thin film to over 1000.degree. C. by bringing a laser to focus on the material, these materials will not follow the law of reciprocity. If an image is separated into two images, each one exposing the recording material separately, the result will be quite different as any heat below the threshold value will dissipate and not add up to the previous exposure. The current invention takes advantage of this property in order to separate a high resolution image into a set of images, each exposed separately, and each one containing only part of the information of the original image. In the following disclosure the word "pixel" is used to describe the smallest feature used as an image. In some cases images do not contain single pixel features, however in this disclosure the smallest feature will be referred to as a pixel. For example, if the smallest feature of an image is 1 micron lines created by stepping a spot of 0.3 micron in increments of 0.1 micron, in this disclosure one pixel will be considered one micron, regardless of the addressability used to create the image.
The behavior of materials obeying linear superposition can be seen in FIG. 1, FIG. 2, FIG. 3 and FIG. 4 depicting prior art. In FIG. 1 a mask 1 containing clear openings 4 is illuminated by light source 2 and imaged on recording material 5 using lens 3. Instead of mask 1 data can be used to modulate light source directly, as shown in FIG. 2 wherein laser 6 is being modulated by data 8. Light from laser 6 is collected by lens 7 and imaged on material 5 using lens 3. The layout of FIG. 2 is more typical of an optical data storage application while FIG. 1 is more typical of microlithography. The principles of the invention apply equally to both configurations as well as numerous other configurations and materials. Referring now to FIG. 3, the effect of limited resolution of the optical system is shown. Mask 1 contains high resolution (single pixels in FIG. 3) patterns. The light distribution next to the mask surface is an accurate copy of the mask, as shown by graph 9. After imaging by lens 3 a light distribution 10 is created on the surface of recording material 5. In order to fully resolve all patterns, graph 10 has to cross the threshold 11 for all features. For example, an isolated clear pixel in a large area (pixel #5 in FIG. 3) may barely cross the threshold 11 as some of the light from pixel 5 spread out. A similar problem occurs between pixels #14, 15, 16 where the lens cannot fully resolve the individual pixels. Threshold 11 is the exposure value causing recording material 5 to change. For example if material 5 is a photoresist threshold 11 is the exposure required to switch it from soluble to non soluble (for negative resist) or the other way (for positive resist). Even if a threshold 11 is found to resolve all pixels the image on recording material 5 will be distorted, as can be seen by comparing 5 to mask 1.
The situation cannot be improved by separating the information on mask 1 into multiple overlapping masks, each one carrying part of the image. This is shown in FIG. 4 mask 1 of FIG. 3 is replaced by two masks: 1A, carrying only the even numbered openings; and 1B for the odd-numbered openings. The two masks create exposure functions 9A and 9B which become respectively 10A and 10B on the surface of recording material 5. Even if the masks are used sequentially, recording material 5 will add up exposures 10A and 10B for an equivalent exposure 10, and end results will be identical to FIG. 3. This was expected as in linear system (i.e. obeying reciprocity law) the response to a function is the sum of the responses to the components of that function, as stated by the principle of linear superposition. It makes no difference if the image data comes from a mask or by direct modulation of the light, as shown in FIG. 2.